1. Field of the Invention
The present invention relates to a gradation correction device for instructing a printing apparatus to print a control strip formed by linearly arranging a plurality of patches on an end portion of a paper sheet, and for generating gradation correction data in accordance with density data obtained by reading the control strip; and a printing system having the gradation correction device.
2. Description of Related Art
In a printing system, stable gradation reproducibility in which the same printed result can be always obtained from the same printing data is required. In particular, in a printing system that performs high-speed printing in large amount, the above stable gradation reproducibility is strongly required.
In a general printing system using an electrophotographic process or the like, the output characteristics are changed due to environmental conditions, change over time in continuous printing, and individual difference between systems. For this reason, generally, by measuring the output characteristics to feed back the measured characteristics to the gradation correction, the output characteristics become constant.
In the printing system that performs the high-speed printing in large amount, in order to maintain the processing ability, it is not preferable to print a test chart separately from a printed document. Therefore, by printing a control strip in a space of a cut-off region of the printed document and reading the control strip, the gradation correction is finely performed.
For example, each patch position in the control strip on the paper sheet is detected in accordance with the relative position to reference marks having a cross shape or the like, which are printed in the vicinity of the control strip (See Japanese Patent Application Publication No. 2003-103762).
In many cases, in the space of the cut-off region, crop marks which are the indication of a cut-off position are arranged. Further, in some cases, another test chart for the correction other than the gradation correction is printed. Therefore, the area that can be allocated for the control strip is restricted.
Meanwhile, it is required to arrange the control strip which is printed so as to have a large area for each patch because of the particle property of the toner used for the printed document, the stability of the printing (the irregularity in the patch) and the like, and which has fine gradation steps and is expressed by a multi-color system having not only the primary colors, but also the secondary colors and the tertiary colors. In order to meet the above requirements, a large paper area is required for the control strip.
For this reason, as disclosed in Japanese Patent Application Publication No. 2003-103762, in a method of additionally arranging the reference marks in the vicinity of each control strip, an extra arrangement space for the reference marks is needed. Accordingly, the area that can be allocated for the control strip is decreased.
Further, because the paper sheet is not precisely fixed and conveyed in the printing of the control strip on the paper sheet and in the reading of the printed control strip, the problems such as the shift of the position, the inclination, the extension or the contraction and the deformation of the paper could be caused for the intended control strip. Accordingly, in order to estimate the position of each patch of the control strip in accordance with the positions of the reference marks with high accuracy, it is required to finely arrange the reference marks. However, as the number of the arranged reference marks is increased, the space for arranging the control strip is not enough. Accordingly, the number of types of the control strips that can be arranged is decreased, and the gradation correction cannot be performed with high accuracy.
Further, there could be a case where the relation between resolution at the time of the printing and the resolution at the time of the reading are not constant, or a case where due to a system configuration, the detailed arrangement positional information of the control strip, the resolution information at the printing or the reading, and the like cannot be obtained or is unknown. It is assumed that there is no information other than the read image data.
However, in case of the method in which the patch is read on the assumption that the patch exists at a predetermined relative position to the reference marks, the information indicating the positional relation between the reference marks and the patch of the control strip, the resolution information, and the like are required. Accordingly, in a system that cannot obtain the above information, the position of each patch cannot be specified.